More than 100 campaign contributors have given money to both men, the Chicago Sun-Times (http://bit.ly/1fz6boF) found in its analysis of campaign contributions since Emanuel ran for mayor and Rauner launched his campaign.

Common donors include Citadel investment firm founder and billionaire Ken Griffin, Groupon co-founder Eric Lefkofsky and Morningstar founder and CEO Joseph Mansueto. Members of Chicago's Crown family, which has an estimated worth of $4 billion, also contributed to both campaigns.

The two men also share the distinction of kicking in the most amount of money into their own campaigns. Emanuel gave himself $1.1 million. To date, Rauner has put $5 million toward his quest for the governor's office; his campaign has raised more than $11 million since last year.

The pace of Rauner's fundraising, even outside of his own investment, has dwarfed that of his competitors. Rauner is facing state Sens. Bill Brady and Kirk Dillard, and state treasurer Dan Rutherford in the March 18 primary.

Emanuel and Rauner come from investment backgrounds.

The mayor left the Clinton White House to become an investment banker in the 1998, and helped put together a major deal involving Rauner's firm's purchase of SecurityLink from SBC Ameritech, worth $500 million.

While Rauner never employed Emanuel, he helped expand Emanuel's personal wealth. The pair also maintains a friendship, with Emanuel paying visiting Rauner's ranch in Montana to fish.

But in the course of campaigning for the GOP nomination, Rauner has, at times, distanced himself. Following a debate at WLS-TV on Thursday, Rauner remarked that he "couldn't remember" the last time he had spoken to Emanuel, noting that it had been "months."

In an interview with the Sun-Times, Rauner said he and Emanuel "disagree on most things"— except school reform.

Rauner touts his conservative fiscal beliefs more than his moderate social views on the campaign trail. He says his history of giving to Democrats and voting in the 2006 Democratic primary doesn't impact his Republican credentials.

"The reality is I've given to 10 times as many Republicans than to Democrats," Rauner said.

"People in this class with many business interests want to cultivate politicians, not necessarily for something that's immediately coming up, but anything that can happen down the road," he told the newspaper. "They tend not to give for the quid pro quo, but for the access when they need it."